Remove the children array for childless nodes.

If data is updated so that some nodes no longer have children, and a
custom children accessor is used, then the children array was not being
removed for such nodes, which meant that hierarchical layouts were
incorrectly taking into account these children.

Fixes #1579.
Rename d3_svg_line{X,Y} to d3_geom_point{X,Y}.

Also, add a number of missing imports: mainly "math/abs".

Fixes #1581.
Fix when Voronoi cell is outside clip extent.

We we’re distinguishing between the case where the cell completely surrounds the
clip extent and where the cell does not intersect the clip extent at all.
Related #1578.
Quantize sites. Fixes #1578.
Minor optimization.
Add missing file; remove obsolete import.
Merge remote-tracking branch 'origin/import-tweaks' into 3.3.8
Merge remote-tracking branch 'origin/hierarchy-children' into 3.3.8
Merge branch '3.3.8'